1. Field of the Invention
The invention is in the field of mechanical seal technology. In particular, the invention is directed to a mechanical seal system having three, four or five mechanical seals adapted to be used with rotating shaft equipment to prevent leakage.
2. Description of the Related Art
Mechanical seals are widely used to prevent leakage in rotary shaft equipment, such as pumps, mixers, agitators, and compressors, in which a rotating shaft passes through an opening in the housing of the equipment. This type of equipment is characterized by a housing in which process fluid is maintained at pressure and therefore prone to leakage where the shaft enters the housing. The mechanical seals are housed in a seal chamber bolted to the face of the pump housing.
A mechanical seal is characterized by a rotating sealing face, a stationary sealing face and a loading mechanism such as a spring, coil or bellows maintaining the sealing faces compressed against each other. The sealing faces are machined smooth—such that surface roughness often may not exceed 2 millionths of an inch. Mechanical seal manufacturers often provide installation parameters which specify the position where an individual seal on the shaft or shaft sleeve must be installed to maintain the proper spring loading tension and proper force on the delicate sealing faces to prevent leakage. The seal system is also provided with secondary seals, such as O-rings, gaskets, wedges, sealing rings and the like compressed around the primary sealing faces to close off secondary leakage paths around the sealing faces.
The mechanical seal systems in use today are single seal and dual seal systems. A dual seal system typically includes two seal assemblies mounted on a shaft with a barrier fluid filling a space between them. Thus, the upstream (most inboard) seal is situated between the process fluid and the barrier fluid. And the downstream seal (most outboard) is between the barrier fluid and the atmosphere. The barrier fluid is maintained at a higher pressure so as to maintain a positive flow of clean barrier fluid across the downstream seal faces, while the upstream seal is lubricated by both the barrier fluid and the process fluid. The barrier fluid is used where the process fluid is corrosive or abrasive and likely to damage the sealing faces over time. Examples of dual seal systems are disclosed in U.S. Pat. No. 4,290,611 to Sedy, which is incorporated by reference in its entirety.
Another type of barrier system is a gas barrier. In these systems, a suitable gas (such as nitrogen) under pressure is used to prevent seepage of process fluid through the mechanical seal to the atmosphere. It is known in the gas barrier systems to provide a sealing face with spiral grooves, such that as an inert barrier gas is provided between the seals, and as the shaft rotates, gas flows into the grooves and creates an opening force between the sealing faces. This force causes the sealing faces to separate a few thousandths of an inch. As a result of reduced or eliminated contact between the sealing faces, wear is reduced and the need to cool the sealing faces is largely eliminated while maintaining the same or better leakage prevention as a “wet barrier seal.” Gas flow to the seal faces is regulated with a gas panel—the barrier gas circulates through the system at a measured pressure and flow rate and ultimately is vented to the process or atmosphere. A gas barrier system having a sealing face with a spiral groove is disclosed in U.S. Pat. No. 5,375,853 to Wasser, et al., which is incorporated by reference for this purpose.
Staged seal assemblies for extreme duty applications are also known, such as described in U.S. Pat. Nos. 3,360,272 to Blom and 7,300,060 to Zagres, which are incorporated by reference. These assemblies are characterized by having a plurality of staged seals disposed in series along a shaft, each provided with a seal bypass. The seal bypass is configured to direct a portion of fluid through a pressure breakdown device around the respective seal. This diversion of process fluid reduces the severity of the duty requirement on each seal.
U.S. Pat. No. 5,901,965 to Ringer et al. is incorporated by reference for its description of bellows and seal ring construction, which is known in the art. The Ringer et al. patent discloses an improvement on a conventional bellows seal, including a stationary seal ring fitted against a stationary seal ring shell and a bellows attached to the side of the seal ring shell opposite the seal ring, in which undesired stresses on the seal ring and seal ring shell are avoided by decoupling the seal ring and the seal ring shell and using a seal element between the seal ring shell and the seal ring to balance load.
One object of the invention is to increase the service life of a mechanical seal system and reduce or eliminate leakage of process or barrier fluid from the equipment housing.
A further object of the invention is to provide a mechanical seal system with three or more mechanical seals that fits existing standard pumps, mixers and other rotating shaft equipment without modification.
A further object of the invention is to provide a mechanical seal system with three, four, five or more mechanical seals which is not a pressure staging system and which does not include a pressure reducing flow path between upstream and downstream seals.
A further object of the invention is to provide a mechanical seal system with extended operational life using individual seals having relatively low pressure rating to accommodate the application.
A further object of the invention is to provide a mechanical seal system having a plurality of barrier fluid sources each providing the same or different barrier fluid to a plurality of seal faces via different fluid pathways. In embodiments, a gas barrier may be used in combination with a liquid barrier fluid.